Surface geochemical exploration for oil and gas: New life for an old technology

Abstract

Editor's Note: The Geologic Column, which appears monthly in TLE , is (1) produced cooperatively by the SEG Interpretation Committee and the AAPG Geophysical Integration Committee and (2) coordinated by M. Ray Thomasson and Lee Lawyer. Surface geochemical exploration for petroleum is the search for chemically identifiable surface or near-surface occurrences of hydrocarbons, or hydrocarbon-induced changes, as clues to the location of oil and gas accumulations. It extends through a range of observations from clearly visible oil and gas seepage (macroseepage) at one extreme to identification of minute traces of hydrocarbons (microseepage) or hydrocarbon-induced changes at the other. Surface geochemical methods have been used since the 1930s, but the past decade has seen a renewed interest in geochemical exploration. This, together with developments in analytical and interpretive methods, has produced a new body of data and insights about geochemical exploration. Many of these developments are summarized in “Hydrocarbon Migration and Its Near-Surface Expression” ( AAPG Memoir 66 ). Geochemical surveys and research studies document that hydrocarbon microseepage from oil and gas accumulations (1) is common and widespread, (2) is predominantly vertical (with obvious exceptions in some geologic settings), and (3) is dynamic (responds quickly to changes in reservoir conditions). The principal objective of a geochemical exploration survey is to establish the presence and distribution of hydrocarbons in the area and, more importantly, to determine the probable hydrocarbon charge to specific exploration leads and prospects. For reconnaissance surveys, seeps and microseeps provide direct evidence that thermogenic hydrocarbons have been generated; that is, they document the presence of a working petroleum system and identify the portions of the basin that are most prospective. Additionally, the composition of these seeps can indicate whether a basin or play is oil-prone or gas-prone. If the objective is to evaluate individual exploration leads and prospects, the results of …